Azodicarbonamidine salts as silver halide fog inhibitors

ABSTRACT

Azodicarbonamidine salts as a class inhibit fog formation in silver halide emulsions when contacted therewith.

United States Patent [19 1 Baldassarri et al.

[ June 25, 1974 AZODICARBONAMIDINE SALTS AS SILVER l-IALIDE FOG INHIBITORS Inventors: Agostino Baldassarri, Savona;

Walter Ferro, Quiliano, both of Italy Assignee: Minnesota Mining and Manufacturing Company, St. Paul, Minn.

Filed: Apr. 18, 1972 Appl. No.: 245,681

Foreign Application Priority Data Apr. 29, 1971 Italy 50035/71 US Cl. 96/76 R, 96/109 Int. Cl G03c 1/34 Field of Search 96/109, 76 R Primary Examiner-Ronald H. Smith Assistant Examiner-Won l-l. Louie, Jr. Attorney, Agent, or Firm-Alexander, Sell, Steldt &

Delahunt 57] ABSTRACT Azodicarbonamidine salts as a class inhibit fog formation in silver halide emulsions when contacted there- I with.

9 Claims, No Drawings and particularly to the use of such compoundsv as fog and spot-inhibiting agents in silver halide photographic emulsions.

Metallic silver appearing in non-exposed areas of a silver halide photographic emulsion layer after development is commonly called fog. This fog may be caused by the action of certain compounds in the emulsion, by the composition and the nature of the developer, by atmospheric oxidation, storage, etc. The type of packaging used for the photographic film can affect fog formation, as can the type of base upon which the silver halide photographic emulsion is spread. It is probable, for example, that a supporting polyester film contributes more greatly to fog in a silver halide emulsion layer than does a supporting cellulose ester film. The storage of emulsions at high temperatures and humidities favors fog formation.

In spite of the fact that fog may generally appear as a uniform blackening of emulsion layers, these layers often show small areas where the fog is darker or lighter than in the surrounding areas. Those small areas are commonly called sensitization or desensitization spots, or more simply black spots" or white spots. Such spots are often related to the presence in the emulsion layer (or in an adjacent layer) of metallic particles, such 'as particles of iron, copper, tin or their derivatives. These particles are, in turn, related to the type of base used (for instance polyester) or are caused by the environment wherein the material has been prepared or used.

The formation of fog and spots in silver halide photographic emulsions continues as a problem in the photo graphic field, and reduction of elimination of this problem is greatly to be desired.

It has now been found that fog and spots (hereinafter collectively called ffog") in silver halide photographic emulsions, can be strongly inhibited by using, as fog inhibitors, an azodicarbonamidine salt. Such compounds preferably have the formula wherein R1, R R and R independently are hydrogen, alkyl or aryl, and X represents an equivalent of anion, such as chloride, nitrate, sulfate or picrate. Formula I can also be written 1 R, IA

the emulsion, maybe placed in a layer adjacent an emulsion layer, or may be introduced into developer baths or pre-developer baths. Preferred azodicarbonamidine salts for this purpose are those defined in Fonnulae I and IA above. In another embodiment, the invention relates to a method for inhibiting fog in a silver halide photographic emulsion which comprises contacting the emulsion with an azodicarbonamidine salt, preferably of the type defined in Formulae I and IA. The ability to inhibit fog in siliver halide emulsions is shared by azodicarbonamidines as a class.

In another embodiment, the invention relates to novel azodicarbonamidine salts which are useful as fog inhibitors for silver halide emulsions and which are characterized by the formula R1 NH NH R3 I i ii- 13 .21: H H/ \R4 wherein R R R and R independently are hydrogen,

alkyl or aryl with the proviso that at least one (and preferably two or more) of R R R and R is different from hydrogen, and X is an equivalent of anion. Formula II cal also be written wherein R R R R and X are as defined next above.

As will be evident from the examples below, azodicarbonamidine salts may be prepared by oxidizing, e. g., with potassium permanganate in aqueous nitric acid, an N-substituted amino guanidine obtained in a known way, e.g., by reacting an N-substituted-S-methyl isothiourea with hydrazine, or the reduction product of an N-subst ituted nitroguanidine with high pressure hydrogen in the presence of a platinum oxide catalyst. In another preparation method, an N-substituted-S-methylisothiourea may be reacted in a suitable solvent, such as pyridine, with an N-substituted amino guanidine; the

' resulting hydrazodicarbonamidine then being oxidized to the corresponding azodicarbonamidine.

As mentioned above, azodicarbonamidine salts as a class have been found to inhibit fog in silver halide photographic emulsions. The choice of groups R R R and R, will vary to some extent the fog inhibiting ca pacity and solubilities of compounds of the invention, and .it is preferred that the alkyl or aryl groups which maybe represented by each of R1, R2, R and R in formulas I, IA, II and II A above be limited respectively to no more than 4 and 14 carbon atoms. Although the selection of the anion X may also effect to a small extent these properties, the selection of X is not critical to the invention. Chloride (e.g., halide), nitrate, sulfate and picrate (particularly the latter) are preferred anions. The compounds of the present invention are -solvent-soluble; e.g., they are soluble in the primarily polar solvents employed in photography, including, for example, water, alcohol, and dimethylformamide. The

invention may be illustrated by reference to the following non-limiting examples:

EXAMPLE 1 N,N'-dimethyl-azodicarbonamidine nitrate Into an open 750 cc flask supplied with mechanical stirrer and a thermometer and placed on a refrigerating bath, were introduced cc of water and 40 cc of 70 nitric acid in which the reduction product of 0.1 moles of N-methyl-nitroguanidine was previously dissolved. (The reduction was performed by known procedures employing high pressure hydrogen in the presence of platinum oxide). The resulting solution was cooled to 5 C and at this temperature was slowly and carefully treated with small portions of powdered potassium permanganate. An exothermic reaction occurred with gas evolution (foaming) and the solution rapidly acquired an increasingly intense color. The permanganate addition was continued until it began to impart a dark coloration which lasted for some seconds; concurrently a yellow crystalline solid formed. The mixture was rapidly filtered and when the product was almost dry, it was washed with 100 cc of ethanol. It was dried under vacuum at 30C. 2.05 g of very small yellow crystals were obtained, with a decomposition point of 162C, formed by N,N-dimethylazodicarbonamidine nitrate. The product, in the form of a yellow powder, was highly soluble in water, soluble in dimethylformamide, insoluble in alcohol, in acetone and in non-polar solvents.

Elemental Analysis:

azodicarbonamidine nitrate in water was treated with a slight excess of a saturated solution of picric acid to cause nearly quantitative separation of N,N'-dimethylazodicarbonamidine picrate in the form of very fine, short needles with an intense red color. The decomposition point was 189C. The product was highly soluble in dimethylformamide, and slightly soluble in ethanol.

Elemental Analysis:

Calculated Found EXAMPLE 3 N,Ndiethyl-azodicarbonamidine nitrate Into a 750 cc covered flask, equipped with mechanical stirrer and a thermometer and placed on a refrigerating bath, were introduced 35 cc of 70% nitric acid and 35 cc of water containing in solution, the product of the reduction (obtained with high pressure hydrogen in presence of platinum oxide) of 0.1 moles of N-ethylnitroguanidine. The solution was cooled to 5C and at this temperature small quantities of potassium permanganate in powder form were added very carefully. The resulting exothermic reaction caused the fonnation of a large amount of foam, and the solution rapidly attained an intense yellow coloration. When almost twothirds of the oxidant was added, a light voluminous yellow solid began to separate out. At the end of the reaction the mixture was rapidly filtered nearly dry; then it was washed with 100 cc of ethanol and dried under vacuum at 35C.

3.1 g of mother-of-pearl-like scales of a light-yellow color, formed by N,Ndiethyl-azodicarbonamidine nitrate, with a decomposition point of 113C were obtained. The solubility was excellent in water, good in dimethylformamide, and slight in alcohol.

Elemental Analysis:

Calculated Found EXAMPLE 4 Elemental Analysis:

Calculated Found EXAMPLE 5 N,N-dipropyl-azodicarbonamidine nitrate Into a 750 cc flask equipped with mechanical stirrer and a thermometer and placed on a refrigerating bath were introduced 35 cc of nitric acid and 25 cc of water containing in solution the reduction product (obtained with high pressure hydrogen in presence of platinum oxide) of 0.1 moles of propylnitroguanidine. The solution was cooled to 5C and at this temperature small amounts of powdered potassium permanganate were very carefully added. An exothermic reaction occurred which produced a large amount of foam, and the solution turned rapidly became intensely yellow. During a two hour period, from about 6.5 to about 7.5 g of powdered potassium permanganate were thus added. Near the end of the reaction, a yellow, crystalline, light and fluffy solid began to separate out. Upon completion of the reaction, the mixture was rapidly filtered to a semi-dry condition and was then washed with ethanol cc) and dried under vacuum at 35C.

Elemental Analysis:

Calculated Found EXAMPLE 6 Elemental Analysis:

' Calculated Found N% 25.65 25.60 v C% 36.65 36.81 H% 3.66 3.74

EXAMPLE 7' N,N'tetramethylhzodicarbonamidine picrate Into a'750 cc flask equipped with mechanical stirrer and a thermometer and placed on a refrigerating bath were introduced 20 cc of 70% nitric acid and 20 cc of water containing in solution the reduction product (obtained withhigh pressurehydrogen in presenceof platinum oxide) of 0.03 moles of N,N'-dimethyl-N- nitroguanidine. The solution was cooled to -3C and at this temperature a small amount of powdered potassium permanganate was carefully added. An exothermic reaction occurred with development of gas. A yellow solution was obtained, which was filtered (maintaining a temperature below 0C) and treated with an excess of a saturated solution of picric acid (0.05

moles). A yellow crystalline solid separated out and.

was collected by filtration, dried under vacuum at 35C and then thoroughly washed with 250 cc of diethylether.

4.5 g of very small, light-yellow needles of N,N'- I solubility of theproduct in dimethylformamide was excellent.

Elemental Analysis:

Calculated Found EXAMPLE 8 N,N'-diphenyl-azodicarbonamidine picrate Into a 750 cc flask equipped with mechanical stirrer and a thermometer and placed on a refrigerating bath were introduced 21.3 g (0.01 moles) of N-phenylamino-guanidine nitrate, and 100 cc of nitric acid and 100 cc of water to form a solution. The solution was cooled to 2C and at this temperature, small amounts of potassium permanganate in powder were added very carefully. An exothermic reaction occurred with gas development. In two hours, 9.7 g of permanganate (0.061 moles) were added. The mixture was rapv idly filtered, maintaining cold conditions, and the filtrate was treated with an excess of a saturated solution of picric acid. A red fluffy solid separated out and was collected on a vacuum filter, dried under vacuum at 30C, and ground twice with 250 cc of ether. The mixture was filtered again. After drying under vacuum at 30C thereremained 12.3 g (equal to 34% of the theoretical yield) of a red' powder, melting at 95C, with decomposition. The solubility of the product was excellent in dimethylformamide, good in alcohol and in acetone, and slight in cold water. The product was then very rapidly crystallized from boiling water. By crystallization an extremely electrifyable powder (e.g., capable of maintaining an electric charge) of a dark yellow color with a melting point of 154C (with decomposition)'was obtained. The solubility was similar to that of the raw product.

N,N-di-m-tolyl-azodicarbonamidine picrate Into a 750 cc flask equipped with mechanical stirrer and a thermometer and placed on a refrigerating bath were introduced 30 cc of 70% nitric acid and 35 cc of water, containing,"in solution, the reduction product (obtained with high pressure hydrogen and platinum oxide) of- 0.05 moles of N-m-tolyl,N'-nitroguanidine. The solution was cooled to -3C and at this temperature, small amounts of powdered potassium pennanganate were added very carefully. An exothermic reaction occurred with gas development, and the solution rapidly became red. After the addition of a rather large quantity of potassium permanganate, a soft, pitchy material of an intense red color began to separate out. After addition of 4.85 g of potassium permanganate, the separated pitchy material, presumably N,NFdi-mtolyl-azodicarbonamidine nitrate, was collected,

washed with small amount of water, dissolved in 30 cc of cold ethanol and treated with an excess of a saturated solution of picric acid. A fluffy red-orange solid was obtained which was separated by filtration, washed with cold water, dried under vacuum at C, and ground twice with 200 cc of ethyl ether in a mortar. After drying, there remained 5.3 g of a yellow red powder of N,N'di-m-tolyl-azodicarbonamidine picrate. M.P. 109C. The solubility of the product was excellent in dimethylformamide, good in alcohol and in acetone, and slight in water.

Elemental Analysis:

N,N'di-a-naphthyl-azodicarbonamidine nitrate Into a 750 cc flask equipped with mechanical stirrer and a thermometer and placed on a refrigerating bath were introduced 40 cc of 70% nitric acid containing dissolved 13.15 g (0.05 moles) of N'-a-naphthyl,N'- aminoguanidine nitrate. The solution was cooled to 2C and at this temperature small amounts of powdered potassium permanagante were carefully added. An exothermic reaction occurred, with gas development, and the solution rapidly turned a dark red. The amount of KMnO. used was 4.85 g (0.6 moles per mole of aminoguanidine). When the reaction was almost completed, a reddish pitchy material began to separate out. After the completion of the permanganate addition, this pitchy material was separated from the mother liquor (a), washed with a small amount of water, and air-dried. It was then ground in a mortar to yield a brown-red powder of N,N'-di-a-naphthylazodicarbonamidine nitrate; M.P. 160C, with decomposition. The solubility of the product was excellent in dimethylformamide and good in ethanol and acetone.

Elemental Analysis:

Calculated Found EXAMPLE 1 l Elemental Analysis:

Calculated Found N% 20.34 l9.69 C% 49.45 49.09 H% 2.9l 3.12

EXAMPLE l2 N,N'-di-(methyl,phenyl)-azodicarbonamidine picrate N-methyl-N-phenyl-N'-aminoguanidine nitrate (11.35 g, 0.05 moles) were dissolved in 30 cc of nitric acid and introduced into a 750 cc flask,,equipped with mechanical stirrer and a thermometer and placed on a refrigerating bath. The temperature was brought to 3C and kept constant while 4.85 g of powdered potassium permanganate were carefully added in small portions over about a one hour period. The addition was performed very carefully to avoid overheating from the resulting exotherm. Gas development was observed. The resulting solution was filtered cold and then treated with an excess of a saturated solution of picric acid. A reddish voluminous solid separated out and was collected by filtration, washed with a small amount of. water, dried under vacuum at 30C, then ground twice with 200 cc of ether, filtered and dried. A reddish-yellow powder of N,N'-di(phenyl,methyl)- azodicarbonamidine picrate (which may be termed N,- N'-dimethyl-N,N-'diphenyl-azodicarbonamidine) was obtained. The melting point was C with decomposition. The yield was 5.6 g, equivalent to 29% of theoretical. Solubility was excellent in dimethylformamide and good in alcohol and in acetone.

Elemental Analysis:

N-ethylN-phenyl-azodicarbonamidine picrate 4.92 g (0.02 moles) of N-ethyl-S-methyl-isothiourea iodide and 5.84 g (0.02 moles) of N-phenyl-N'- aminoguanidine iodide, and 20 cc of pyridine were introduced into a 250 cc flash equipped for reflux. The mixture was heated to the boil. After a few minutes it was clearly evident from the odor of methylmercaptan that the reaction had begun. Heating was continued until the development of methylmercaptan ceased. The mixture (aftercooling) was poured into 200 cc of ethyl ether and the resulting pitchy material was washed several times with ether. The same pitchy material (consisting primarily of N-phenyl-N-ethyl,hydrazodicarbonamidine iodide) was finely powdered and suspended in 50 cc of methylene chloride. A solution of 0.02 moles (3.1 g) of N-chloro-benzotriazole in 20 cc of methylene chloride were added dropwise to the suspension with stirring and cooling to 0C. The mixture turned intensely red and an orange pitchy solid separated out. The mixture was filtered under vacuum and washed carefully with a small amount of cold ethanol (about 3 cc). The solid was dissolved in cc of etha-' Elemental Analysis:

Calculated Found Azodicarbonamidine salts are useful for prevention of fog in silver chloride, silver bromide and silver iodide emulsions and in emulsions comprising mixtures of such silver halides (such as, for example, silver bromoiodide emulsions). The emulsions of the invention may contain various synthetical polymers as colloidal liquids in substitution for, or in addition to the commonly employed gelatin. Exemplary of such polymers are dextrane, polyvinylalcohol, polyvinylpyrrolidone, partially hydrolized polyvinylacetate, polyethylacrylate, polymethylmetacrylate and polyamides. The emulsions of the invention may be chemically sensitized by remelting with naturally activated gelatin, by addition of chemical sensitizers such as thiosulfate, allylthiourea, thiocyanates, thiosulphonates, etc., by employment of salts of noble metals (for example gold salts), etc. The emulsions may contain spectral sensitizers such as cyanine and mercocyanine dyes and may contain couplers, surface active agents, hardeners, stabilizers, antifog agents, plasticizers, anti-oxidants, development accelerators and, in general, the various additives commonly used' in the production of silver halide photographic emulsions. Such emulsions may be spread upon any suitable support such as polystyrene, polyester, cellulose acetate, polycarbonate, paper, glass, etc. The azodicarbonamidine salts of the present invention are Y particularly suitable for the prevention of fog usually occurring when a polyester film support (for example, polyethyleneterephthalate) is used or when unusually rapid processes for the spreading and drying of the emulsions are used.

The incorporation of azodicarbonamidine salts in sil- -ver halide emulsions may be conveniently accomplished by adding them to the emulsions during preparation thereof. Even though it is preferable that the azodicarbonamidine salts be incorporated into silver halide emulsion layers, fog and spots are avoided or minimized if the salts are at least put into contact with ide layer or in an adjacent layer will vary depending 6 about 2.5 millimoles of an azodicarbonamidine salt such as those exemplified above per gram-atom of silver.

The novel compounds of Examples 1-13 have been evaluated photographically together with previously known azodicarbonamide salts representing further examples of such salts useful as fog-inhibitors according to the present invention. These compounds are azodicarbonamidine nitrate (Example 14), azodicarbonamidine sulfate (Example 15), azodicarbonamidine chloride (Example 16) and azodicarbonamidine picrate (Example l7).

EXAMPLE 18 The solubility of compounds l-l7 in common solvents used in photography (water, ethyl alcohol and dimethylformamide (DMF)) at room temperature (about 20C) was measured. The results are schematically reported in Table 1 wherein indicates a solubility greater than 1 per cent and a solubility less than 0.05%.

TABLE 1 Compound H O C H OH DMF l l l 1 I l 1 l l l l l l l EXAMPLE 19 A silver halide photographic emulsion of high sensitivity, containing 1.8 mole of Agl and 98.2 mole of AgBr, with a ratio of silver/gelatin of 1.6 was divided into several parts. One part was spread, without further treatment, on a cellulose triacetate base. To the other parts were added the compounds indicated in Table 2 in the reported quantities, and the resulting emulsions were similarly spread on a cellulose triacetate base. The resulting film samples were then exposed to tungsten light and developed for 5 or 10 at 20C in a Ferrania R-l4 developer of the following composition (Ferrania R-l4 developer):

Metol Water to Table 2 reports the results of sensitometric test performed on fresh film samples and on film samples stored respectively for 15 hours at 72C, and at 30% RH.

TABLE2 fie y v EFL H Fog Relative S lvent N0. AgX Fresh 151172c 30 mm. Sensitivity *rise 23 30 22 32 100 14 100 22 27 20 26 117 11,0 14 500 07 05 05 14 71 14 2500 06 06 06 13 36 15 100 1a 26 20 26 135 1-1,o 15 500 14 24 15 22 85 15 2500 06 12 05 11 25 16 100 09 15 07 14 71 11 16 500 09 14 07 14 71 16 2500 13 04 11 36 17 100 is 26 22 2s 89 C H OH E 17 500 13 26 22 2s 89 17 2500 14 22 14 22 89 I 1 100 15- 24 20 26 100 11 0 1 500 15 22 16 24 96 1 2500 14 20 14 22 79 2 100 19 26 20 23 112 DMF 2 500 15 24 20 26 100 2 2500 09 13 17 56 3 100 27 31 23 34 110 11 0 3 500 23 29 23 32 v 110 3 2500 18 2s 19 2s 9s 4 100 21 24 22 26 112 DMF 4 500 24 16 24 96 4 2500 12 15 12 19 79 5 100 24 32 22 94 107 11,0v 5 500 19 2s 19 29 91 5 2500 I8 28 17 27 91 6 100 25 29 23 34 91 DMF 6 500 21 27 21 33 59 6- 2500 19 27 19 33 59 7 100 15 21 14 0 1-1 011 7 500 08 16 05 16 10.5 7 2500 05 15 05 16 16 s 100 11 1s 03 19 63 canon 13 500 09 1s 08 17 s 2500 05 16 03 10 11, 9 100 10 26 0s 1s DMF 9 500 07 1s 06 17 22 9 2500 07 1a 06 17 1o 10 100 25 33 25 35 96 c n ou 10 500 23 29 24 29 79 10 2500 18 25 15 19 79 11 100 24 32 27 34 9s c,1-1,0H 11 500 22 29 24 29 so 11 2500 17 26 16 1s 79 12 100 15 23- 09 25 79 c,H,0H 12 500 08 19 06 20. 2s 12 2500 03 07 04 0s 5 13 100 06 16 26 35 34 CZHBOH 13 500 04 10 12 20 25 13 2500 04 10 0s 1s ,14

EXAMPLE 20 compounds 1nd1cated 1n Table 3 1n the reported quantie-J9 t ether n i hswe eeside re ect y the ties, and the resulting emulsions were spread on a cellulose triacetate base. Samples of the resulting films werethen exposed to tungsten light and developed for 3' at 20C in Ferrania R-l4 developer (See Example 19').

AfiL ifirQqntmus 9"!P"." uM/M. Fog a i e lven No. AgX Fresh 15 h 72C 30% R.H. Sensitivity used 17 100 12 1s 90 17 500 12 18 15 2o 90 Q11 0" 17 2500 10 15 10 15 90 1 100 11 17 14 18 100 1 500 10 15 11 27 95 11 0 1 2500 09 14 1o 15 so 2 100 13 1s 14 19 115 2 500 10 17 14 18 100 DMF 2 2500 06 09 07 12- 58 3 100 19 22 17 23 10s 3 500 16 20 16 21 108 11,0 3 2500 12 19 13 1s 95 4 100 15 17 15 18 110 4 500 10 17 11 17 95 DMF 4 2500 08 10 0s 13 80 5 100 17 22 15 24 110 5 500 13 20 13 20 91 H20 5 2500 12 12 12 19 90 6 100' 17 20 16 24 95 6 500 15 19 15 23 61 DMF 6 2500 13 19 13 23 59 v7 100 10 15 10 14 7 500 06 11 I 03 11 11 0 11 011 7 2500 03 1o 03 11 2 8 100 03 13 06 13 65 s 500 06 12 06 12 45 CZHSOH s I 2500 03 10 02- 07 12 9 100 07 1s 06 13 4s 9 500 05 13 04 12 25 DMF 9 2500 05 12 04 12 .12 10 100 17 22 113 24 91; 10 500 16 20 16 19 80 0 11 011 10 2500 13 28 11 13 so 11 100 17 22 19 A 23 100 c gn 11 500 15 20 17 1 29 s5 11 2500 12 1s 11 13 so 12 100 11 16 06 18 so 12 500 06 14 04 14 I 31 Cg-[ 0H 12 2500 02 06' 03 06 6 13 100 04 11 1s 24 35 13 500 03 07 09 14 213 c.11 0 13 2500 03 07 O6 13 15 EXAMPLE 21 What we claim 1s:

A high sensitivity silver halide emulsion containing 1.8 mole of Ag! and 98.2 mole of AgBr was divided into several portions. To these portions were added respectively the compounds reported in Table 4. The resulting emulsion samples were then spread on a polyester support'previously thinly coated with gelatin containing iron powder. Samples of the resulting films were developed in Ferrania R-l4 developer (Example 19) for 3 at 20C. The capacity of the compounds to inhibit spot formation was evaluated by counting the number of the spots per unit area in several areas of 1. A silver halide photographic emulsion containing a fog inhibiting amount of an azodicarbonamidine salt.

2. The emulsion of claim 1 wherein said azodicarbonamidine salt is characterized by the formula H E R4 wherein R R R and R independentlyare hydrogen, alkyl or aryl, and X represents an equivalent of anion.

3. The emulsion-of claim 2 wherein the anion of said azodicarbonamidine salt is chloride, nitrate, sulfate or picrate.

4. The emulsion according to claim 1 wherein said azodicarbonamidine salt is contained therewithin in a concentration of from about 0.1 .to about 2.5 millimoles per gram atom of silver.

5. The silver halide photographic emulsionaceording to claim 1 wherein said azodicarbonamidine salt is a salt of N,N' :dimethyl-azodicarbonz midine. 7 7 MN 6. The photographic emulsion according'to claim 5 wherein said azodicarbonamidine salt is :the nitrate or picrate salt.

7. A photographic element which includes a silver halideemulsion layer. said emulsion layer containing a 7 15 16 fog inhibiting amount of an azodicarbonamidine salt, or sion layer. v said emulsion layer having an adjacent layer containing 9. The photographic element according to claim 7 wa fog inhibiting amount of an azodicarbonamidine salt. herein said azodicarbonamidine salt is in a layer adja- 8. The photographic element of claim 7 wherein said Cent Sald S'lver hahde -SIQW YQ azodicarbonamidine salts is incorporated in said emul- 

2. The emulsion of claim 1 wherein said azodicarbonamidine salt is characterized by the formula
 3. The emulsion of claim 2 wherein the anion of said azodicarbonamidine salt is chloride, nitrate, sulfate or picrate.
 4. The emulsion according to claim 1 wherein said azodicarbonamidine salt is contained therewithin in a concentration of from about 0.1 to about 2.5 millimoles per gram atom of silver.
 5. The silver halide photographic emulsion according to claim 1 wherein said azodicarbonamidine salt is a salt of N,N''-dimethyl-azodicarbonamidine.
 6. The photographic emulsion according to claim 5 wherein said azodicarbonamidine salt is the nitrate or picrate salt.
 7. A photographic element which includes a silver halide emulsion layer, said emulsion layer containing a fog inhibiting amount of an azodicarbonamidine salt, or said emulsion layer having an adjacent layer containing a fog inhibiting amount of an azodicarbonamidine salt.
 8. The photographic element of claim 7 wherein said azodicarbonamidine salts is incorporated in said emulsion layer.
 9. The photographic element according to claim 7 wherein said azodicarbonamidine salt is in a layer adjacent said silver halide emulsion layer. 